Making up for forgetting to recite Surah Al-Fatihah by offering a fifth Rak`ah

Q 1: In one of the Masjids (mosques) during the ‘Isha’ (Night) Prayer, when the Imam (the one who leads congregational Prayer) got up to offer the fourth Rak‘ah (unit of Prayer), he remembered that he had not recited Surah Al-Fatihah in the third Rak‘ah. So, he abrogated the third Rak‘ah and considered the fourth to be in its place. The Ma’mums (persons being led by an Imam in Prayer) did not know what he had done, so when he stood up to perform the fourth Rak‘ah, they thought it was the fifth. They became divided; one group stood up and followed the Imam, others remained sitting waiting for him to sit down to say the Taslim (salutation of peace ending the Prayer) after him, and yet some others sat down, said the Tashahhud (a recitation in the sitting position in the second/last unit of Prayer), and the Taslim while the Imam was standing. After the Imam had said the Taslim, he told the Ma’mums what had happened and told those who did not follow him to stand up and offer the last Rak‘ah, which they had thought was the fifth, and perform Sujud-ul-Sahw (Prostration of Forgetfulness). Please give us your legal opinion as to who was correct in this situation, and may Allah reward you.

A: What the Imam said was correct. Those who did not follow the Imam in offering this last Rak‘ah for completion of the four-Rak‘ah Salah had to offer it after he said the Taslim and also perform Sujud-ul-Sahw. (Part No. 6; Page No. 24) Their Salah is correct - In sha’a-Allah (if Allah wills) - because they did not deliberately intend not to follow the Imam, it was because they thought that he was offering a fifth Rak‘ah by mistake.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.